Synthetic polymers such as acrylic polymers have, for the most part, replaced naturally occurring materials such as animal glues, starches and vegetable gums in many adhesive applications. The key to this replacement was the development and use of tackifier resins in the formulation with synthetic polymers, because they give the system the ability to wet the substrate surface and thus form a strong bond. Without tackifiers in the adhesive formulation, the synthetic polymers have low tack and peel adhesive performance.
Various resinous materials have been suggested as tackifiers for synthetic polymers. Examples of materials suggested include rosin, disproportionated rosin, rosin esters such as glycerol esters of rosin and pentaerythritol esters of rosin, aliphatic hydrocarbon resins from petroleum, aromatic petroleum resins, dicyclopentadiene (DCPD) resins, terpene resins, terpent/phenol resins and cumarone/indene resins.
Tackifiers are resins that can be considered as high viscosity solvents for the base polymer. The term tackifier therefore, cannot be given to any material unless we relate that material to the product that it tackifiers.
Acrylic polymer compositions have long been used as adhesive compositions, particularly in pressure sensitive adhesive (PSA) compositions. Common PSA compositions include a high-molecular-weight acrylic base polymer and a tackifying resin and when required, plasticizer, stabilizer, antioxidant, filler, coloring agent and the like. PSA compositions should have a high initial tack so that the adhesive surface merely needs to be contacted and pressed to the surface to achieve bonding. There should be little or no requirement to hold the adhesive and substrate in position for any significant time while a bond develops.
In developing adhesive formulations one seeks the optimum combination of several adhesive properties and since it is not generally possible to achieve the optimum for every property it is necessary to obtain the best balance of properties for the particular use in mind. In e.g. PSA compositions which are used in the label industry one tries to optimise the loop tack and peel and at the same time provide an adhesive whose component will not migrate during storage.
Nowadays mainly rosin derivatives, such as disproportionated rosin acids, and hydrocarbon resins are commercially used as tackifiers for adhesive compositions comprising acrylic polymers. The rosin derivatives have been preferred due to better compatibility with acrylics.
Initiatly most important were the solvent acrylics, but environmental, health and safety needs of industry have resulted in dramatic growth in water borne adhesive compositions comprising acrylic polymers. With increasing desirability of water borne systems and often their demonstration of significant performance enhancements with tackifier resins, new tackifier resins suitable for these systems are being sought.
U.S. Pat. No. 5,120,781 discloses tackifiers for hot melt adhesive compositions. The tackifiers are produced by reacting rosin and a polyhydric alcohol under conditions effective to substantially complete the esterification reaction. The resulting reaction product is then reacted with an aromatic dibasic acid providing a tackifier with a content of rosin above 78 weight %.
U.S. Pat. No. 4,981,939 refers to a binder for preparing toners which are used for developing electrostatic images formed in the process of electrography. The toner comprises as the main component a polyester composed of a polybasic acid component and a polyhydric alcohol component and a rosin modified polyhydric alcohol. The rosin modified polyhydric alcohol is produced by reacting a polyhydric alcohol having three or more hydroxyl groups with a rosin resulting in a polyester network (cross-linking).
WO 9942508 relates to low viscosity aromatic polyester polyols prepared by reacting a phthalic acid based material with diethylene glycol, a higher functional polyol having an average functionality of greater than two and a long chain alkyl acid, ester or oil. The resulting aromatic polyester polyol is a branched polyester.
U.S. Pat. No. 3,438,918 refers to rubber compositions obtained by reacting polyhydric alcohols, polybasic acids and oils.
Thus one advantage of the present invention is to provide a tackifier which is compatible with water borne pressure sensitive adhesive compositions and hot melt pressure sensitive adhesive compositions.
The further advantage with the present invention is the improvement of overall adhesive properties of adhesive compositions and specifically improving the properties of loop tack and peel and at the same time provide an adhesive (composition) whose component(s) will not migrate during storage.
A still further advantage with the present invention is to provide a polyester tackifier comprising a minimum amount of rosin derived from natural sources thereby reducing the cost of production, i.e. a polyester which is capped at both ends with a monofunctional compound (a polyester containing not more than two rosin residues).
Another advantage with the present invention is to provide a straight chain polyester resin being solid or liquid.
Still further advantages of the present invention are disclosed by the following specification.